builder.md

Builder Pattern

Problem

We need to create a complex object that is hard to configure.

Solution

The Builder pattern encapsulates the construction logic of complex objects in its own class. It defines an interface to configure the object step by step, hiding the implementation details.

Example

Let's imagine that we have to build a system that keeps track of the components of a computer.

class Computer
  attr_accessor :display
  attr_accessor :motherboard
  attr_reader :drives
  
  def initialize(display=:crt, motherboard=Motherboard.new, drives=[])
    @motherboard = motherboard
    @drives = drives
    @display = display
  end
end

The creation of a Computer object can get really complex, as the Motherboard, for example, is a whole object:

class CPU
  # Common CPU stuff...
end

class BasicCPU < CPU
  # Lots of not very fast CPU-related stuff...
end

class TurboCPU < CPU
  # Lots of very fast CPU stuff...
end

class Motherboard
  attr_accessor :cpu
  attr_accessor :memory_size

  def initialize(cpu=BasicCPU.new, memory_size=1000)
    @cpu = cpu
    @memory_size = memory_size
  end
end

So, the process of building a Computer can be really tedious:

# Build a fast computer with lots of memory...
motherboard = Motherboard.new(TurboCPU.new, 4000)

# ...and a hard drive, a CD writer, and a DVD
drives = []
drives << Drive.new(:hard_drive, 200000, true)
drives << Drive.new(:cd, 760, true)
drives << Drive.new(:dvd, 4700, false)
computer = Computer.new(:lcd, motherboard, drives)

It can be made much simpler by encapsulating the construction logic in a class:

class ComputerBuilder
  attr_reader :computer

  def initialize
    @computer = Computer.new
  end

  def turbo(has_turbo_cpu=true)
    @computer.motherboard.cpu = TurboCPU.new
  end

  def display=(display)
    @computer.display=display
  end

  def memory_size=(size_in_mb)
    @computer.motherboard.memory_size = size_in_mb
  end

  def add_cd(writer=false)
    @computer.drives << Drive.new(:cd, 760, writer)
  end

  def add_dvd(writer=false)
    @computer.drives << Drive.new(:dvd, 4000, writer)
  end

  def add_hard_disk(size_in_mb)
    @computer.drives << Drive.new(:hard_disk, size_in_mb, true)
  end
end

builder = ComputerBuilder.new
builder.turbo
builder.add_cd(true)
builder.add_dvd
builder.add_hard_disk(100000)